Every bolting application presents a unique set of operating conditions. However, all bolting applications require uniform compression and accurate residual bolt load. This requirement stems from the fact that a major cause of failure in bolted joints is an incorrect preload. While in service, bolts that are improperly preloaded for a specific set of operating conditions may experience cyclic stresses that induce fatigue failures. Correctly preloading a bolt results in improved bolt fatigue life, increased joint rigidity, and the elimination of bolt relaxation, joint separation, and joint slippage. Therefore, proper preloading can eliminate catastrophic failures and increase service life.
To achieve the required preload, an accurate means of determining the tensile force applied to the bolt must be employed. Of the variety of preloading techniques currently in use, torquing, thermal tensioning, and hydraulic tensioning are the most common. Torquing is conventionally used for bolt preloading because of its perceived simplicity and reduced cost. However, the difficulty of quantifying the frictional forces that must be overcome during torquing results in significant deviations (e.g., up to .+-.30%) from the desired preload. Moreover, torsional stresses introduced during torquing may cause distortion, thread galling, stress cracking, and bolt fatigue. Thermal tensioning (i.e., heating and cooling the bolt to be tensioned) is a complicated procedure that requires special tools and components to achieve a desired preload. Moreover, the method is laborious, time consuming, expensive, and may present a significant safety hazard.
Hydraulic bolt tensioning applies a direct axial load to the bolt to be tensioned. In operation, the hydraulic bolt tensioner is threadably engaged to the bolt, a hydraulic pressure is applied and the bolt is preloaded, the nut is tightened, and the pressure is released. Because hydraulic tensioning eliminates the frictional and torsional forces encountered with torquing, it results in more accurate and consistent bolt preloading. In addition, hydraulic bolt tensioners are generally smaller, lighter, and easier to use than torquing tools. Therefore, their compact design and lightweight enables them to be used in many applications where limited space would prohibit the use of bulky torque wrenches or impact tools. Moreover, hydraulic bolt tensioning does not require heat, hammering, or loud noises.
Referring to FIG. 1A, a conventional fixed-size hydraulic bolt tensioner 10 includes a bridge 15 which rests upon a surface (not shown) through which a bolt 20 to be tensioned passes. A body 25, mounted on top of the bridge 15, receives a piston 30. The piston 30 includes threads 35 which are of the same size as the threads 40 on the bolt 20. The piston 30 and the bolt 20 are threadably engaged in preparation for applying a preload to the bolt 20. The preload is created by a hydraulic pressure source (not shown), which may be connected to the hydraulic bolt tensioner 10 via a threaded pressure coupling 45. The threaded pressure coupling allows a pressure to be applied at the interface 50 between the piston 30 and the body 25. The pressure at the interface 50 is substantially maintained by a pair of seals 55 interposed between the piston 30 and the body 25. The delivery of a pressurized hydraulic fluid to the interface 50 results in relative movement of the piston 30 with respect to the body 25 and the application of an axial load along the center line of the bolt 20. Once a desired preload is achieved, a nut 60 may be tightened (e.g., either by hand or with tools), the pressure released, and the hydraulic bolt tensioner 10 disengaged from the bolt 20.
Where tensioning of different sized bolts is required, a plurality of fixed-size hydraulic bolt tensioners, one for each size bolt to be tensioned, has usually been employed. Alternatively, referring to FIG. 1B, a variable-size hydraulic bolt tensioner 65 has been used. The primary difference between a variable-size and a fixed-size hydraulic bolt tensioner resides in the use of an adaptor 70 with the piston 30 to vary the size of bolt which the hydraulic bolt tensioner may engage. The adaptor 70 includes a threaded internal surface 75 for engaging the threads 40 of a bolt 20 to be tensioned. Because the external surface 80 of the adaptor 70 is smooth, the internal surface 85 of the piston 30 likewise does not include threads (compare FIG. 1A).
Variable-size hydraulic bolt tensioners operate in much the same manner as described above for fixed-size hydraulic bolt tensioners. However, the insertion of the adaptor 70 into the piston 30 requires that the adaptor shoulder 90 bear the reaction force normally borne by the threads of the piston (compare FIG. 1A). The force bearing capability of the adaptor is proportional to the bulk of the adaptor shoulder 90 and the piston wall 95. Because the preloads applied during hydraulic bolt tensioning can be as high as 1,000,000 lbs., the bulk of the adaptor may consume a significant portion of the volume of the central cavity 100 of the piston 30. Therefore, the range of sizes to which a given variable-sized hydraulic bolt tensioner may be adapted is limited. Moreover, the required bulk of the adaptor increases material costs, and therefore the equipment expenses associated with variable-size hydraulic bolt tensioning. In addition, the smooth surface interface of the external surface 80 of the adaptor 70 and the internal surface 85 of the piston 30 may result in the adaptor 70 becoming a high velocity projectile should the bolt 20 or threads 40, 75 fail during tensioning. Such a projectile creates a significant safety hazard to operators of hydraulic bolt tensioners. Moreover, the adapters are only compatible with variable-size hydraulic bolt tensioners and cannot be used to adapt fixed-size hydraulic bolt tensioners to different sized bolts. Therefore, in order to tension bolts of varying size, an operator currently owning fixed-size hydraulic bolt tensioners must choose between purchasing a wide variety of fixed-size hydraulic bolt tensioners or a variable-size hydraulic bolt tensioner with a wide variety of adapters.